226 THE BLASTOPORE. 



the thickened axial portion of the hypoblast in the region of the 

 embryo becomes separated from the lateral part as the notochord. 

 Very shortly after the formation of the notochord, the hypoblast 

 grows in from the two sides, and becomes quite continuous 

 across the middle line. The formation of the notochord takes 

 place from before backwards ; and at the hinder end of the 

 embryo the notochord is continued into the mass of cells which 

 forms the axis of the primitive streak, becoming therefore at 

 this point continuous with the epiblast. The notochord in 

 fact behaves exactly as did the axial hypoblast in the earlier 

 stage. 



In comparison with Lacerta (pp. 203 205) it is obvious that the axial 

 hypoblast and the notochord derived from it have exactly the same relations 

 in Mammalia and Lacertilia. In both they are continued at the hind end of the 

 embryo into the epiblast ; and close to where they join it, the mesoblast and 

 epiblast fuse together to form the primitive streak. The difference between 

 the two types consists in the fact that in Reptilia there is formed a passage 

 connecting the neural and alimentary canals, the front wall of which is con- 

 stituted by the cells which form the above junction between the notochord 

 and epiblast ; and that in Mammalia this passage which is only a rudi- 

 mentary structure in Reptilia has either been overlooked or else is absent. 

 In any case the axial junction of the epiblast and hypoblast in Mammalia 

 is shewn by the above comparison with Lacertilia to represent the dorsal lip 

 of the true vertebrate blastopore. The presence of this blastopore seems to 

 render it clear that the blastopore discovered by Ed. van Beneden cannot 

 have the meaning he assigned to it in comparing it with the blastopore of 

 the frog. 



Kolliker adduces the fact that the notochord is continuous with the axial 

 cells of the primitive streak as an argument against its hypoblastic origin. 

 The above comparison with Lacertilia altogether deprives this argument of 

 any force. 



At the stage we have now reached the three layers are defi- 

 nitely established. The epiblast (on the view adopted above) 

 clearly originates from epiblastic segmentation cells. The hypo- 

 blast without doubt originates from the hypoblastic segmenta- 

 tion spheres which give rise to the lenticular mass within the 

 epiblast on the appearance of the cavity of the blastodermic 

 vesicle ; while, though the history of the mesoblast is still ob- 

 scure, part of it appears to originate from the hypoblastic mass, 

 and part is undoubtedly formed from the epiblast of the primi- 

 tive streak. 



